Mishkat al-Masabih #1886
Fasting
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَنْ عَمَّارِ بْنِ يَاسِرٍ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: «مَنْ صَامَ الْيَوْمَ الَّذِي يُشَكُّ فِيهِ فَقَدَ عَصَى أَبَا الْقَاسِمِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ» . رَوَاهُ أَبُو دَاوُدَ وَالتِّرْمِذِيُّ وَالنَّسَائِيُّ وَابْنُ مَاجَهْ والدارمي
‘Ammar b. Yasir said, "He who fasts on a day about which he is doubtful has disobeyed Abul Qasim"*
*i.e., the Prophet.
Abu Dawud, Tirmidhi, Nasa’i, Ibn Majah and Darimi transmitted it.
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About Mishkat al-Masabih
This hadith is #1886 from Mishkat al-Masabih, in the Book of Fasting.